Hi Ros; I have tried all sorts of bird scarers and I cannot really claim that any was the ultimate deterrent. I think that any percieved success was not really due to the bird scarers at all. Pigeons are very tolerant of human activity and if they are hungry they will be in, as soon as you walk away. Local farmers use loud gas powered scarers to limit the crop damage and scarecrows can be quite innovative sometimes. I had a flight of fancy the other day, when I was in my creative mood, and conceived a solar powered, air operated, inflatable scarecrow, which would lie deflated most of the time and rapidly inflate when the air pump was charged up. However; I need help with the engineering 
Pete